The Mega Auction in the IPL allows teams to retain only 3 out of the possible 20 to 25 players in the team. All the rest of the players are released back into the auction where teams can bid for them.
This usually happens every 3-5 years (decided by BCCI). Recently, there are discussions going on whether or not BCCI should have another Mega Auction next year. Some team owners have expressed their concerns over this type of auction being done often as it takes away their ability to develop a long term core group and playing environment.
In the years where there is no Mega Auction, teams are allowed to release players into the mini auction where there are a limited players available to be sold to different teams. Trade windows are also opened for teams to trade players according to internally finalised contracts.